1. Field of the Invention
The invention relates to an electrophotographic printing system and, more particularly, to an electrophotographic printing system having a moistureless electrophotographic development cartridge having an electrostatically operative toner transfer system and an associated humidity-absorbing drying means.
2. Description of Related Art
Printers provide a means of outputting a permanent record in human readable form. Most printers may be characterized as utilizing an impact printing technique or a non-impact printing technique. In impact printing techniques, an image is formed by striking an inked ribbon placed near the surface of the paper. Impact printing techniques may be further characterized as either formed-character printing or matrix printing. In formed-character printing, the element which strikes the ribbon to produce the image consists of a raised mirror image of the desired character. In dot-matrix printing, the character is formed as a series of closely spaced dots which are produced by striking a provided wire or wires against the ribbon. By selectively striking the provided wires, any character representable by a matrix of dots can be produced.
Non-impact printing is often preferred over impact printing in view of its tendency to provide higher printing speeds as well as its better suitability for printing graphics and half-tone images. Non-impact printing is also relatively noise free. One form of non-impact printing is generally classified as ink jet printing. In this technique, ink is force, most commonly under pressure, through a tiny nozzle to form a droplet. The droplet is electrostatically charge and is attracted to an oppositely charge platen located behind the sheet of paper. Using electrically controlled deflection plates similar to those in a CRT, the trajectory of the droplet can be controlled to hit a desired spot on the paper. Unused drops are deflected away from the paper into a reservoir for recycling the ink. Due to the small size of the droplet and the precise trajectory control, ink jet printing quality can approach that of formed-character impact printing.
Another non-impact printing technique is generally referred to as electrophotographic printing. In this technique, a controller, for example, a microprocessor associated with a computer system, turns a small laser or other light source on and off at a very rapid rate which readily exceeds millions of times per second. Reflection means, for example, a multifaceted polygonal mirror, reflects the light stream off a facet thereof and sweeps the light stream across a negatively charged print drum. The reflected light discharges portions of the surface of the negatively charged print drum, thereby producing a latent image of the drum. As the print drum rotates, it is dusted with negatively charged toner in the form of small particles. The toner particles stick only to the discharged areas. When negatively charged paper contacts the drum, the toner particles are attracted to the discharged image ares, thereby forming the desired image. The image is then fused to the paper by a combination of heat and pressure. As the finished page is produced, the drum is cleared of its electrical charge, cleaned and recharged for a next cycle.
Various techniques have been utilized to supply toner to a photosensitive surface, for example the surface of a print drum, where a latent image has been formed. In cascade type developing devices, a developer material which is comprised of carrier material and toner particles is cascaded onto the photosensitive surface from a position above the surface thereby developing a latent image previously formed on the photoreceptor surface into a visible toner powder image. The use of cascade type developing devices is often undesirable due to the size, slow process speed and somewhat reduced print quality of such devices, particularly when the reproduction of half-tones and solid areas is desired.
Many other techniques utilize magnetic attraction to supply toner to the photosensitive surface of a print drum. For example, U.S. Pat. No. 3,985,436 to Tanaka et al. discloses an electrophotographic copying system in which stationary bar magnets attract developing material consisting of magnetizable carrier material such as iron particles and electroscopic toner particles onto the surface of a cylinder to form magnetic brush bristles. As the cylinder rotates, the brush bristles rub lightly against the latent image formed on the surface of the rotating drum to develop the latent image into a visible toner powder image.
While electrophotographic printing techniques which utilize magnetic developers produce high quality images at relatively fast processing speeds, there remain several drawbacks to such systems. In particular, the brushing action on a print drum produced by a developer roller carrying a developer tends to wear itself out as well as wear out the print drum. Numerous solutions to this problem have been proposed. For example, U.S. Pat. No. 4,538,896 to Tajima et al. describes an electrophotographic copying system where a hopper supplies a magnetic component to a magnetic roller. In turn, the magnetic roller transfers the toner to a photosensitive drum.
Other solutions avoid the use of magnetic developers entirely. For example, U.S. Pat. No. 4,100,884 to Mochizuki et al. discloses an apparatus for developing an electrostatic image on a photosensitive member in which a one component non-magnetic toner is supplied to a developer roller. As the surface of the developer roller is formed of an elastic rubber having good adherence and retention qualities for fine powders, the toner is readily attached to the surface of the developer roller as a toner layer. In the event that the deposited toner is not level, a leveling member is included to provide a uniform thickness for the toner layer. The apparatus disclosed by Mochizuki et al. further includes a triboelectric charging member which charges the retained toner to a given polarity. As the electrostatic attraction is greater than the toner retaining capability of the developer roller, the charged toner will then be deposited on the image bearing member.
Other drawbacks that occur with conventional electrophotographic printers stem from the effects of changes in relative humidity. The presence of moisture within the internal module or modules of an electrophotographic printer, most commonly caused by changes in relative humidity, contaminates the internal wiring and adversely affects toner consistency and flowability, as well as toner transfer efficiency and efficiency in negatively charging the print drum. Because moisture also tends to cause paper that is fed through the device to become limp, electrophotographic printers have typically had to include bulky supporting apparatus to ensure proper feeding of the paper despite the effects of moisture. Further, the presence of moisture adversely affects the efficient operation of the thermal fusing element by forcing heat to be dissipated in removing moisture from within the device prior to its being applied to its primary function of fusing the toner to the paper.
The presence of moisture within an electrophotographic printer negatively affects the means by which a latent image is developed on a photoconductive media through a process generally known as a fermi level energy mechanism. Moisture within the development module of an electrophotographic printer or copier degrades the integrity and fermi energy of the development materials. The reduced attractability of the toner that results from the presence of moisture aggravates the similarly-reduced efficiency of charging the print drum, thereby resulting in output images that are degraded in terms of image density, background density and that contain toner spread around image text lettering. No solution to the problems associated with the presence of moisture within an electrophotographic printer or copier has, as yet, been proposed.